From the people who unearthed the Sony rootkit debacle, Sysinternals has released an updated version of their RootkitRevealer software designed to scan for and reveal any rootkits installed on your computer.

RootkitRevealer is an advanced patent-pending rootkit detection utility. It runs on Windows NT 4 and higher and its output lists Registry and file system API discrepancies that may indicate the presence of a user-mode or kernel-mode rootkit.

As the software that first discovered the Sony rootkit, RootkitRevealer has got some good credentials. If you're worried that you might have a rootkit installed, check it out. If you don't know what a rootkit is, the RootkitRevealer page also has a good explanation.